Plastic double-walled, foam-core conduit having solid plastic inner and outer tubes and foamed core have been disclosed in published articles, e.g., "Modern Plastics", November, 1978 issue, pages 78-80.
Such pipes or conduits have been coextruded using a primary extruder for the foam-core layer or tube and a satellite extruder for the outer and inner skin tubes. The coextrusion die uses a feed block design with dual gate valves to control flow to the inner and outer skin tubes which are formed simultaneously around a foam-core layer or tube from the primary extruder in a common die.
Foam-core conduits have the advantage of being less dense without sacrificing properties, hence, savings in raw material and handling costs are realized.
As the art progessed the coextrusion process was found to have certain limitations, in particular, the density of the foam-core has been kept in the range of about 0.50 to 0.90 grams per cubic centimeter so that it would not collapse during its coextrusion with the inner and outer skins. The solid skin material is generally extruded at about 5.degree. to 30.degree. F. higher temperature than the foam-core layer, hence, when the three streams passed through the common profile die, the foam layer can be collapsed by the hotter skins or flow differentials.
There has developed a need for even lower density foam core pipe to further reduce raw material and energy costs consistent with optimized physical properties.
It is an objective of the present invention to provide a continuous process for the extrusion forming of double-walled, foam-core conduits wherein the foam-core layer has a density of 0.015 to 0.4 grams per cubic centimeter.
U.S. Pat. No. 3,379,221 relates to double-wall plastic conduits having a porous cementious fill material as a core layer incorporated after said double wall profile has been formed. U.S. Pat. No. 3,845,184 relates to a process for extrusion forming higher density foamed plastic extrudates having an annular profile and thin skins such that the tubular extrudate is self-supporting. The known prior art then relates to double-walled plastic conduits having high density foamed cores prepared by coextrusion.